What to consider when doing 10G over CWDM ?

Passive CWDM over a single or duplex single mode fiber is quite common to extend your overall bandwidth. In these days when 1G Ethernet was sufficient the main limiting factor was the power budget of your optical transceiver and the attenuation/kilometer of your fiber (e.g. a G.652 has 0,20dB/km @ 1550nm [1]). Based on this it is possible to transmit up to 200km with a single transceiver ( S.1612.41.xD having a power budget of 41dB) with the speed of 1G Ethernet.
Today, the majority of new installed links operate at 10G Ethernet. Here CWDM becomes a little bit more tricky. Beside the fiber attenuation you also have to include the chromatic dispersion of your fiber in your calculation. Chromatic dispersion is the time variance which occurs to a single pulse of your signal - the signal is stretched on the fiber transmission path due the dispersion characteristics of the transporting fiber (Chromatic Dispersion is "the spreading of a light pulse per unit source spectrum width in an optical fibre caused by the different group velocities of the different wavelengths composing the source spectrum." [2])
Chromatic dispersion exists always but it becomes more and more important the higher the link speed is. If we do our math and take a 15km G.652 fiber [1] we will end up with the following result for the dispersion (chromatic dispersion value * link length):
wavelength chromatic dispersion fiber attenuation
1310nm 0 ps/nm 5,25 dB
1490nm 200 ps/nm* 3,6 dB
1550nm 270 ps/nm 3 dB
1610nm 330 ps/nm 3,45dB
* calculation based on the Zero Dispersion Wavelength (λ0): 1304nm ≤ λ0 ≤ 1324nm and Zero Dispersion Slope (S0): ≤ 0.089 ps/(nm2 * km)
[1];  Dλ = S0 / 4 * ( λ - (λ04 / λ3))

matching this with the dispersion tolerance of the CWDM lasers used in various CWDM SFP+
partnumber description powerbudget dispersion tolerance max. distance limiting factor
P.1696.10.x DFB based CWDM SFP+ 13dB 0 - 200ps/nm 8 to 30km dispersion
P.1696.14.x EML based CWDM SFP+ 14dB 0 - 800ps/nm 40km attenuation
P.1696.23.x EML based CWDM SFP+ 23dB 0 - 1600ps/nm 70km attenuation

we come to the following conclusion for your 15km G.652 fiber:
  1. you can only use CWDM channel 1270nm to 1490nm when taking a DFB based CWDM SFP+
  2. you can use the EML based CWDM SFP+ with 14dB power budget and have the full 1270nm to 1610nm CWDM support.
To sum it up. While doing your 10G planing you should know the total fiber length and the fiber type (e.g. G.652 or G.655). Take the recommended parameters from ITU-T for your dispersion and attenuation calculation. If you even know the vendor and product type of the fiber than take the datasheet because the actual fiber has mostly a better performance than the standards claim.
Disclaimer: I did not use any insertion loss of the CWDM Mux/Demux in my calculation - a rule of thumb for a 8-channel CWDM Mux is 2dB insertion loss per end.
[1] G.652 compliant Corning fiber datasheet
[2] ITU-T G.652 Characteristics of a single-mode optical fibre and cable
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